Namecheap tells me I could buy that domain for 98 cents, are you sure you own that domain?

i using domain from my local country name tenten.vn when i deploy to heroku i add custom domain it work and now when i move to railway it show Waiting for DNS update too long

You need to buy the domain chloestudio.shop from either cloudflare or namecheap (not tenten), Railway does not buy domains on your behalf.

I mean I bought the domain chloestudio.shop from a domain provider in our country and it is tenten.vn. After that, I configured DNS according to railway's instructions below, but my application could not be accessed via the domain name chloestudio.shop
https://docs.railway.app/guides/public-networking#add-a-custom-domain

As per my first message, the domain chloestudio.shop is still available for purchase, you need to buy it from either namecheap or cloudflare.

Regardless, tenten.vn does not support the root level CNAME that Railway would have you use.